1. Field of the Invention
This invention relates to a technique for converting multitone image data into pseudo-tone data represented by the dot on-off state.
2. Description of the Related Art
Ink jet printers, liquid crystal displays, and other image/video display devices that represent an image by creating dots on a display medium enjoy widespread use as output devices for computer-created images, images shot with digital cameras, and so on. In order to represent a multitone image using an ink jet printer or other image display device of this kind, the multitone image data for the image must first be converted into pseudo-tone data represented by the dot on-off state. While a number of methods have been proposed for performing this conversion, a typical method that is widely used at present is the so-called error diffusion method.
The error diffusion method involves deciding the dot on-off state for a given pixel, and diffusing the tone representation error (hereinafter, this error shall be termed tone error) into unprocessed neighboring pixels. When deciding the dot on-off state for an unprocessed pixel, the dot on-off state is decided upon so as to cancel out error diffused from neighboring pixels in this manner. Taking the case of an ink jet printer by way of example in the following description, if it has been decided for example to produce a dot in a highlight area of high brightness level in the image, large tone error will be produced for that pixel. By diffusing this error into unprocessed neighboring pixels to make dot formation less likely in the neighboring pixels, the error can be cancelled out overall. Conversely, in a shadow area having low brightness in an image, if it has been decided to not form a dot, large tone error will result. By diffusing the tone error created by this pixel into neighboring pixels to make dot formation more likely in the neighboring pixels, the error can be cancelled out overall.
In contrast to an ink jet printer, with a video display that produces an image display composed of luminescent spots, such as a liquid crystal display, large tone error will result from a decision to not form a dot in a highlight area, or a decision to form a dot in a shadow area. As regards liquid crystal displays, since the relationship between dot on-off status and production of tone error is the reverse of that for an ink jet printer, they can be considered in terms of an ink jet printer. To avoid excessively complicated description, the following description shall assume an ink jet printer; however, it would apply analogously to a liquid crystal display or similar device.
In the error diffusion method, the neighboring pixel range into which tone error will be diffused, and the weighting factor employed when diffusing the error into neighboring pixels, are precalculated in a numerical table termed an error diffusion matrix. Tone error is diffused into neighboring pixels in accordance with this matrix. To achieve higher picture quality, it has been proposed to use different error diffusion matrices depending on tone value of the image data (for example, Unexamined Patent Application 7-226841).
For example, since, in a highlight area, it is desirable for dots to be distributed sparsely (i.e., with dots spaced far apart), it is effective to employ an error diffusion matrix having a large diffusion range. By so doing, when a dot is produced on a given pixel, the tone error resulting therefrom is diffused into a wide neighboring range, dot formation can be made less likely in the neighboring area of the dot, so that dispersion of dots can be improved. In other words, in a highlight area, in order to make formation of additional dots unlikely in proximity to a pixel having a dot formed thereon, it is desirable to employ an error diffusion matrix whereby tone error produced by a given pixel maybe diffused in to a wide range neighboring the pixel. Similarly, in a shadow area, as it is desirable in terms of picture quality for pixels having no dots formed thereon to be distributed sparsely, the use of a large error diffusion matrix is preferred. By so doing, tone error resulting when a dot is not formed may be diffused into a wide neighboring range, thereby improving dispersion of dots to give better picture quality. In contrast to the above, in middle tone areas containing both dot-on pixels and dot-off pixels formed at relatively high density, dot dispersion is not appreciably worsened by the use of a small error diffusion matrix having a narrow diffusion range, and has the advantage of actually reducing the number of pixels into which error must be diffused, producing a commensurate speedup in processing for conversion of the image data.
It will thus be apparent that, in terms of picture quality, for highlight areas and shadow areas it is desirable to use as large an error diffusion matrix as possible, so as to diffuse tone error over a wide range. However, there have been encountered instances in which, despite the use of an error diffusion matrix having a wide diffusion range, dots nevertheless are formed relatively close together so that picture quality is not improved to the extent expected.
The present invention is intended to address the issues in the prior art discussed above, and has as an object to provide a technique for effective improvement of dot dispersion in highlight areas and shadow areas.